Gravity-compensation type accelerometer and process for producing such an accelerometer

ABSTRACT

An accelerometer includes a seismic mass which is subjected to a force when accelerated. The seismic mass is connected to a support by a mechanical connection which can bend under the influence of the force. A detector is provided to determine the acceleration induced in the seismic mass by the force. Compensation can be provided for the force exerted on the mass due to gravity. The mechanical connection includes at least one component which provides compensation and induces in the mechanical connection a prestress counteracting the force exerted on the mass by gravity.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a gravity-compensation typeaccelerometer. Compensating for the effect of gravity on the seismicmass of an accelerometer gives increased sensitivity to variations inacceleration.

The invention applies particularly to small apparatuses. The nature ofthe accelerometer makes it suitable for construction using mechanical,micromechanical or microelectronics techniques (for examplemicromachining).

The main field of application for an accelerometer using the presentinvention is for studying the movement or behavior of milieus subject togravity (for example seismology).

2. Discussion of the Background

The invention thus makes it possible to design gravity-compensationaccelerometers where known types of one-piece accelerometer do not allowfor this compensation. Accelerometers using known techniques aredescribed in, for example, M. Ueda, H. Inada, Y. Mine and K. Sunago:"Development of micromachined silicon accelerometer" in the SumitomoElectric Technical Review, No. 38 of June 1994, pages 72-77 and inMichael E. Hoenk: "Small inertial measurements units-sources of errorand limitations on accuracy" in the review SPIE, vol. 2220, pages 15-26.

The most common method for measuring acceleration consists in notdirectly measuring the acceleration itself but rather the force Fapplied to a mass M due to the effect of the acceleration γ in question.According to the basic law of motion F=M.γ, if the value of M is known,F can be measured and a value for the acceleration obtained.

The most common type of acceleration sensor thus consists of an inert,or seismic, mass, generally supported by one or more springs. When themass is subjected to variations in acceleration, it moves and thesprings are distorted. The system returns to its initial position assoon as the force due to the acceleration is canceled.

A horizontal acceleration sensor in the rest state is not sensitive toany disruptive effect. On the other hand, a vertically-sensitiveaccelerometer is subject to a minimum force equivalent to that ofgravity, F=M.g, where g is the gravitational constant.

This minimum force due to gravity is inconvenient when attempting tomeasure very slight vertical accelerations (less than 10⁻⁶ G). It istherefore important in this situation to compensate for the effort dueto gravity with a force tending in the opposite direction to thatexerted by gravity. At the present time there are two classes ofprocesses for compensating for the force of gravity:

processes using a source of electric power. An electromagnetic orelectrostatic field maintains the seismic mass in suspension. Suchprocesses require complex servo systems,

processes using the return force of a spring. The mass is maintained ina state of equilibrium, suspended by a pre-distorted spring.

There are also hybrid systems that use a combination of electrostatic orelectromagnetic forces together with the return force of a spring. Anexample of this type of system is described in Shi Jung Chen and KuanChen: "The effects of spring and magnetic distortions on electromagneticgeophones" in J. Phys. E. Sci. Instrum. 21 (1988), pages 943-947.

These techniques have other disadvantages.

In electrostatic or electromagnetic apparatuses, the presence of anelectronic servo system can generate interference noise that isincompatible with the desired sensitivity. Moreover, purelyelectrostatic compensation methods produce unstable systems that aredifficult to servo-control.

Vertically-sensitive sensors where the effect of gravity on the seismicmass is compensated for by a spring are currently produced by assemblinga variety of mechanical parts. This type of sensor is described in, forexample, E. Wielandt and G. Streckeisen: "The leaf-spring seismometer:design and performance" in Bulletin of Seismological Society of America,Vol. 72 No. 6; pages 2349-2367, December 1982. By virtue of theirconstruction, this type of apparatus does not have a very high Q qualityfactor. This structural parameter is related to the density of Browniannoise S of the apparatus using the following relation: ##EQU1## where:ω=pulse

ω.sub.γ =resonance pulse

k_(b) =Boltzman constant

T=temperature

M=mass of seismic mass

For more information on this relation, please refer to the article"Small inertial measurements units-sources of error and limitations onaccuracy" cited above. The relation shows that S is inverselyproportional to Q and M.

To preserve levels of Brownian noise that do not disrupt measurement,present apparatuses have a significant mass M. However, this solutionlimits miniaturization of the assembly. Of accelerometers with the bestperformance characteristics (for example, those capable of detectingvariations of a few nano-G below 1 G), the smallest weigh severalkilograms and have volumes measured in tens of cubic centimeters.

In conclusion, vertically-sensitive accelerometers are either veryinsensitive or are heavy and bulky. Miniaturization of ahigh-performance device would require a reduction in size of the seismicmass to increase the quality factor. This may be achieved by using amaterial with a high quality factor such as, for example,mono-crystalline silicon for manufacturing the sensor assembly. However,building a compact apparatus that includes a spring fastened to theseismic mass presents some technical difficulties. In practice it isdifficult to fasten small mechanical parts such as the spring and theseismic mass to one another using mechanical means such as screws orcement without creating areas where internal friction is significant,causing damping phenomena prejudicial to the quality factor. Moreover,the spring must retain a high degree of flexibility since thisflexibility influences the sensitivity of the sensor.

SUMMARY OF THE INVENTION

The technique for compensating for the effect of gravity used in thepresent invention is related to the group of sensors in which theseismic mass is supported by a spring. This solution has the advantageof reducing the noise interference produced by the servo systemsrequired by other types of apparatus. The proposed compensationtechnique is based on the principle of a leaf spring produced byprestressing one surface of a component (for example a strut) bearingthe seismic mass.

The invention therefore relates to an accelerometer comprising a seismicmass capable of being subjected to a force induced by the accelerationto be measured, the seismic mass being fastened to a support bymechanical connecting means capable of bending under the influence ofsaid force, means of detection being provided to determine theacceleration induced in the seismic mass by the force, means ofcompensation also being provided to compensate for the force exerted onthe seismic mass by gravity, characterized in that the mechanicalconnecting means include at least one component constituting said meansof compensation and inducing in such mechanical connecting means aprestress that counteracts the force exerted on the seismic mass bygravity.

The means for compensating for the force exerted on the seismic mass bygravity may consist of a surface layer applied to the mechanicalconnecting means, this surface layer being applied to one surface of themechanical connecting means such that it counteracts the force exertedon the seismic mass by gravity and consisting of a material having theproperties required to exert stress on said mechanical connecting means.

For these purposes said material exerting the stress may be selectedfrom the group of materials consisting of chromium, molybdenum,tungsten, an alloy thereof or a PZT-type ceramic.

The means for compensating for the force exerted on the seismic mass bygravity may consist of two surface layers applied to two surfaces of themechanical connecting means, one surface layer consisting of a materialthat induces a tensile stress and the other layer inducing a compressivestress, the combination of these two layers inducing a stress gradientin the thickness of the mechanical connecting means that counteracts theforce exerted on the seismic mass by gravity.

In this case, the two layers may consist of thin films of molybdenumapplied using different techniques to produce contrary stresses.

In a different embodiment, the means for compensating for the forceexerted on the seismic mass by gravity may consist of modifying thesurface of the mechanical connecting means inducing said prestress inthe mechanical connecting means.

This surface modification may advantageously consist of doping thesurface of the material constituting the mechanical connecting means.

Since the mechanical connecting means in this situation are made ofmono-crystalline silicon, one of the surfaces of the mechanicalconnecting means may be doped using a dopant selected from the group ofmaterials consisting of phosphorus, boron, xenon, titanium, arsenic andargon.

Surface doping may be carried out on opposite surfaces of the mechanicalconnecting means, each surface being doped with a different dopant, oneof which induces a elongation stress while the other induces acompressive stress resulting by inducing a stress gradient in thethickness of the mechanical connecting means that counteracts the forceexerted on the seismic mass by gravity.

If the mechanical connecting means are made of silicon, one surface ofthe mechanical connecting means may be doped with boron while-the othersurface is doped with argon.

The mechanical connecting means may consist of one or more struts.

This type of design makes it possible to build the seismic mass, themechanical connecting means and the support as a single assembly. Incontrast with one-piece accelerometers constructed using knowntechniques, the invention therefore makes it possible to build aone-piece gravity-compensation type accelerometer.

The invention therefore also covers a process for manufacturing anaccelerometer comprising a seismic mass connected to a support bymechanical connecting means capable of bending under the effect of aforce induced in the seismic mass by the acceleration to be measured,means of compensation also being provided to compensate for the forceexerted on the seismic mass by gravity, characterized by the followingsteps:

masking of one of the main surfaces of a substrate, the first surface,to delimit the seismic mass and the support,

engraving the first surface of the substrate in the direction of theother main surface of the substrate, the second surface, leaving amembrane between the bottom of the engraving and the second surface,this engraving delimiting the seismic mass and the support,

masking the second face of the substrate in such a way as to mask thesupport, the seismic mass and the mechanical connecting means,

engraving the second surface of the substrate to open unmasked areas ofthe membrane,

processing at least part of the surface of the mechanical connectingmeans to induce a prestress counteracting the force exerted on theseismic mass by gravity, thereby constituting said means ofcompensation.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described below in more detail. The othercharacteristics and advantages disclosed in the following descriptionare given as examples and are not intended to be limiting. Thedescription refers to the attached drawings, in which:

FIG. 1 is a side view of a gravity-compensation type accelerometer builtaccording to a first variant of the invention,

FIG. 2 is a partial perspective view of a gravity-compensation typeaccelerometer built according to a second variant of the invention,

FIG. 3 is a partial perspective view of a gravity-compensation typeaccelerometer built according to a third variant of the invention,

FIG. 4 is a top view of a strut supporting a seismic mass coated with astress deposit and engraved as an accelerometer according to theinvention,

FIG. 5 is a perspective view of an accelerometer according to theinvention under construction.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, accelerometers will be described thatinclude a seismic mass that is distinct from the component connecting itto a support, this connecting component consisting of one or morestruts. This is not restrictive on the invention which applies equallywhere the seismic mass is not different from the strut or is part of thestrut.

FIG. 1 is a simplified illustration of the principle of the invention asapplied to a vertically-sensitive accelerometer. It shows a seismic mass1 of mass M connected to a support 2 by means of a strut 3. The seismicmass 1 is thus overhanging in relation to support 2. Overlooking themass of strut 3, the center of gravity of seismic mass 1 is sensitive toforce F to which the seismic mass is subjected following an accelerationg.

To compensate for the effect of gravity on seismic mass 1, the uppersurface 4 of strut 3 is processed to induce a prestress which causes aforce to be exerted on the seismic mass that compensates for the forceinduced on the seismic mass by gravity.

This surface stressing of the strut can be obtained in various ways. Itmay be obtained by depositing a thin film on the surface of the strut orby modifying the surface of the strut significantly enough to produce aneffect of stress. The strut thus processed tends to bend, exerting aforce opposed to that of gravity. The stress conditions must be suchthat the end of the strut exerts an upward compensating force as shownby the curved arrow in FIG. 1. If the stress is sufficiently intense,this force counteracts the force of gravity.

Where a layer is deposited on the strut, the deposited material chosenmust be a metal or metal alloy or a material known for presentingstresses such as certain piezoelectric materials such as the ceramicdescribed by the formula Pb(Zr_(x) Ti_(1-x))O₃ also known as PZT. Thepreferred materials are chromium, molybdenum, tungsten or an alloy ofone of these metals.

Modifying the strut is understood to mean using suitable means to modifythe surface area of the material constituting the strut itself. Thismeans may be by doping the surface of the strut to a minimal depth. Inthis situation it is advantageous to select a dopant suitable for thematerial constituting the strut in order to obtain a bimetallic stripeffect between the doped and undoped sections of the strut. If the strutis made of mono-crystalline silicon, one of the following elements canbe used as the dopant phosphorus, boron, xenon, titanium, arsenic orargon.

The solution proposed by the invention thus makes it possible to designa spring connected to the seismic ass forming an assembly that can besmall in size without having to resort to mechanical connecting meansmade up of separate components.

FIG. 1 illustrated the simplest version of an accelerometer, i.e. havingonly one strut. The invention nevertheless applies to accelerometershaving several struts.

The principle of the invention may be applied to accelerometers havingtwo, four or eight struts to limit the degree of freedom (in translationand in rotary motion around the center of gravity) of the verticallydisplaceable seismic mass. In these embodiments such struts are arrangedin opposing pairs or opposing groups of four.

FIG. 2 shows this type of accelerometer. The seismic mass 10 isconnected to the support 11, which has been leveled off around theseismic mass to simplify the figure, by four struts 12 arranged inopposing pairs. A suitable surface deposit 13 covering the four struts12 as well as the seismic mass 10 has been applied to compensate for theforce of gravity acting on the seismic mass 10.

FIG. 3 is the same type of illustration as FIG. 2 and shows aneight-strut accelerometer. This accelerometer can be obtained by bondingtwo structures 21 and 22 each having four struts (i.e. both structuresare of the type shown in FIG. 2). Bonding may be achieved using knownbonding or cementing methods such as, for example, the method describedin the article "Application of oxygen plasma processing to silicondirect bonding" by O. Zucker, W. Langheinrich, M. Kulozik and H. Goebelin the review "Sensors and Actuators" A. 36, 1993, pages 227-231.

Since the surface deposits must exert their force to compensate for theforce of gravity, deposits 23 and 24 are applied to the surfaces of thestruts that will be uppermost once the accelerometer has been assembled.

The planar stresses exerted by the thin layer deposited on the surfaceof a strut also induce transverse distortion of the strut perpendicularto the direction of the first distortion. The resulting curveaccentuates the moment of inertia of the strut and therefore thestiffness of the system, which affects the sensitivity of the sensor. Ifthis phenomenon is found to be potentially too disruptive, it may beremedied in several ways.

A first solution consists in applying a discontinuous deposit to thestrut as shown in FIG. 4, which is a partial top view showing anaccelerometer built according to the invention. The seismic mass 31 willbe recognized, connected to the support 32 by strut 33. The uppersurface of the strut 33 is covered not with a continuous deposit butwith parallel lines 34 oriented between the seismic mass and the support32. It has been shown that in this form the thin film deposited tends topartially free the component from stresses acting perpendicularly to thelines. For more information on this subject, see "Analyse pardiffraction des rayons X, de l'evolution des contraintes residuellesassociees a la gravure de lignes dans un depot mince de tungstene CVDsur substrat de Si" (Analysis using X-ray diffraction of the developmentof residual stresses associated with lines engraved in a thin deposit ofCVD tungsten on an Si substrate) by L. Maniguet, M. Ignat, M. Dupeux,J.J. Bacmann and P. Normandon in "Revue de Metallurgie-CIT/Science etGenie des Materiaux", September 1993, page 1109.

A second solution consists in using the intrinsic anisotropy of thestresses in certain thin metallic films to orient the maximum force inthe most favorable direction, i.e. between the seismic mass and thesupport. These phenomena of intrinsic anisotropy have been describednotably in P. Gergaud and J. J. Bacmann: "Internal stress tensordetermination in molybdenum and molybdenum-carbon thin films depositedby D.C. magnetron sputtering" published in "Materials Science Forum"vol. 133-136, 1993, pages 873-878.

Combining these two solutions makes it possible to bring all thesephenomena into play and, in some cases, to eliminate the transversestress.

A third solution consists in making one or more longitudinal slots inthe strut, so that the slots run from the seismic mass to the support.Where the means of compensating the force of gravity consist of asurface layer deposited on the strut, the layer is slit, preferablyalong its entire thickness, using a pattern similar to that shown inFIG. 4.

Surface stress may be exerted by a deposit or processing used to createa stress gradient within the strut. The use of thin films makes itpossible to design a two-layer system, i.e. referring to FIG. 1, a thinfilm 4 and a thin film 5 deposited parallel to one another on eitherside of the strut. The upper thin film should exert a tensile stresswhile the lower thin film exerts a compressive stress. Some materialsapplied in thin films (for example molybdenum) exert opposing types ofstress depending on how they are applied. The article "Internal stressesin sputtered chromium" by D. W. Hoffman and J. A. Thornton published inthe review "Thin Solid Films", 40 (1977) pages 355-363 describes thisphenomenon in the case of chromium.

Moreover, in the field of microelectronics technology, doping techniquesare used to obtain compressive or tensile stresses depending on thenature of the element implanted and the conditions under whichimplanting is carried out. This is discussed in the article "Parallelstress and perpendicular strain depth distributions in 001! siliconamorphized by ion implantation" by R. Fabbri, M. Servidori and A. Zani,published in the review J. Appl. Phys. 66 (10), 15 November 1989, pages4715-4718. For example, doping with boron produces tensile stresses insilicon while doping with argon produces compressive stresses in thesame material.

Where the material used to create the upper thin film (and exerting atensile force) is the same as that used to create the lower thin film(and exerting a compressive force), the thermal expansion coefficient ofthe thin films is close, even identical, and the system is thusunaffected by temperature. Where thin films are deposited parallel toone another on either side of the strut, molybdenum is the preferredmaterial. Where thin films are created by doping the material composingthe strut (e.g. silicon), even though different dopants are used on thetwo surfaces, the thermal expansion coefficients of the two layers arevery close because the substrate material to which the dopants areapplied is identical.

An example of an accelerometer constructed according to the presentinvention will now be described. The embodiment chosen uses amicroelectronics technique to build a four-strut accelerometer of thetype shown in FIG. 2. The sensor is made of <100>-orientation silicon.After chemical cleaning, the silicon is covered with a mask which may bea layer of silicon nitride Si₃ N₄. By using conventionalphotolithographic processes, an opening of an area delimiting theseismic mass is created in the mask. The silicon is then subjected to ananisotropic engraving process, for example in a bath of potassiumhydroxide KOH (see, for example, the article "Development ofmicromachines silicon accelerometer". cited above). The engraving timemust be long enough to produce a thin silicon membrane around theseismic mass.

FIG. 5 shows the result obtained on completion of this phase of theprocess. In this figure, the initial substrate 40 is shown incross-section. The section cuts through the seismic mass 41 giving anidea of the thickness of the membrane 42 remaining around the seismicmass 41.

Surface 43 of the substrate 40 located on the membrane side is thencovered with a layer of silicon oxide SiO₂. This coating is again openedso as to demarcate the sides of the struts and the perimeter of theseismic mass. A physical engraving technique (plasma engraving) in agaseous mixture of boron trichloride BCl₃ and chlorine Cl₂ is then usedto eliminate the silicon from the unmasked areas of the membrane. Oncompletion of this operation the seismic mass stands free of thestructure and is only supported by the struts. The remaining layer ofsilicon oxide is removed by plasma engraving in a gaseous mixture oftrifluoromethane CHF₃ and oxygen O₂.

The stress thin film is then applied using magnetron cathode sputtering.The parameters for creating the molybdenum thin film are regulated toproduce tensile stresses in the material.

This type of structure can thus be made of silicon or quartz using themicromachining techniques used in microelectronics. This productionmethod enables a one-piece silicon assembly to be produced (i.e. anassembly in which all the components of the sensor are machined in asolid substrate) with a high quality factor. It is therefore possible toimagine a lower mass and consequently more compact structure.

Where a bimetallic strip type strut is used, the force designed tocompensate for the force of gravity is significantly affected by theshape of the strut. The present inventors have found that a bimetallicstrip that is rectangular when viewed in the direction of the force ofgravity gives less satisfactory compensation for the force of gravitythan a triangular shape where the base of the triangle is embedded inthe support and where the apex is joined to the seismic mass.

Given the simplicity of producing an accelerometer according to thepresent invention, series production of the sensors can be contemplatedwith consequent reduction in cost-price. The invention could be usedparticularly for making small-size seismometers suitable for use inoil-drilling operations.

We claim:
 1. Accelerometer comprising a seismic mass capable of beingsubjected to a force induced by the acceleration to be measured, theseismic mass being connected to a support by mechanical connecting meanscapable of bending under the influence of said force, means of detectionbeing provided to determine the acceleration induced in the seismic massby the force, means of compensation also being provided to compensatefor the force exerted on the seismic mass by gravity, wherein themechanical connecting means include at least one component constitutingsaid means of compensation while inducing in the mechanical connectingmeans a prestress that counteracts the force exerted on the seismic massby gravity;wherein in the means for compensating for the force exertedon the seismic mass by gravity consist of a surface layer deposited onthe mechanical connecting means, this surface layer being applied to onesurface of the mechanical connecting means in such a way that itcounteracts the force exerted on the seismic mass by gravity andconsisting of a material having the properties necessary to exert astress on said mechanical connecting means; and wherein the surfacedeposit is composed of parallel lines oriented between the seismic massand said support.
 2. Accelerometer comprising a seismic mass capable ofbeing subjected to a force induced by the acceleration to be measured,the seismic mass being connected to a support by mechanical connectingmeans capable of bending under the influence of said force, means ofdetection being provided to determine the acceleration induced in theseismic mass by the force, means of compensation also being provided tocompensate for the force exerted on the seismic mass by gravity, whereinthe mechanical connecting means include at least one componentconstituting said means of compensation while inducing in the mechanicalconnecting means a prestress that counteracts the force exerted on theseismic mass by gravity;wherein the means for compensating for the forceexerted on the seismic mass by gravity consist of a surface layerdeposited on the mechanical connecting means, this surface layer beingapplied to one surface of the mechanical connecting means in such a waythat it counteracts the force exerted on the seismic mass by gravity andconsisting of a material having the properties necessary to exert astress on said mechanical connecting means; and wherein the surfacedeposit has an intrinsic stress anisotropic, the surface deposit beingapplied to accentuate the direction which the prestress is exerted. 3.Accelerometer comprising a seismic mass capable of being subjected to aforce induced by the acceleration to be measured, the seismic mass beingconnected to a support by mechanical connecting means capable of bendingunder the influence of said force, means of detection being provided todetermine the acceleration induced in the seismic mass by the force,means of compensation also being provided to compensate for the forceexerted on the seismic mass by gravity wherein the mechanical connectingmeans include at least one component constituting said means ofcompensation while inducing in the mechanical connecting means aprestress that counteracts the force exerted on the seismic mass bygravity;wherein the means for compensating for the force exerted on theseismic mass by gravity consist of a surface layer deposited on themechanical connecting means, this surface layer being applied to onesurface of the mechanical connecting means in such a way that itcounteracts the force exerted on the seismic mass by gravity andconsisting of a material having the properties necessary to exert astress on said mechanical connecting means; and wherein the surfacedeposit is split by at least one slot oriented between the seismic massand said support.
 4. Accelerometer comprising a seismic mass capable ofbeing subjected to a force induced by the acceleration to be measured,the seismic mass being connected to a support by mechanical connectingmeans capable of bending under the influence of said force, means ofdetection being provided to determine the acceleration induced in theseismic mass by the force, means of compensation also being provided tocompensate for the force exerted on the seismic mass by gravity, whereinthe mechanical connecting means include at least one componentconstituting said means of compensation while inducing in the mechanicalconnecting means a prestress that counteracts the force exerted on theseismic mass by gravity;wherein the means for compensating for the forceexerted on the seismic mass by gravity consist of two surface layers onsaid mechanical connecting means, these surface layers being depositsapplied to opposite surfaces of the mechanical connecting means, one ofthe two deposits consisting of a material that induces a tensile stressand the other deposit inducing a compressive stress, the combination ofthese two deposits inducing a stress gradient in the thickness of themechanical connecting means that counteracts the force exerted on theseismic mass by gravity.
 5. Accelerometer according to claim 4, whereinthe two deposits consist of thin layers of molybdenum applied usingdifferent techniques such as to cause them to exert opposing types ofstress.
 6. Accelerometer comprising a seismic mass capable of beingsubjected to a force induced by the acceleration to be measured, theseismic mass being connected to a support by mechanical connecting meanscapable of bending under the influence of said force, means of detectionbeing provided to determine the acceleration induced in the seismic massby the force, means of compensation also being provided to compensatefor the force exerted on the seismic mass by gravity, wherein themechanical connecting means include at least one component constitutingsaid means of compensation while inducing in the mechanical connectingmeans a prestress that counteracts the force exerted on the seismic massby gravity;wherein the means for compensating for the force exerted onthe seismic mass by gravity consist of a surface layer deposited on themechanical connecting means, this surface layer being applied to onesurface of the mechanical connecting means in such a way that itcounteracts the force exerted on the seismic mass by gravity andconsisting of a material having the properties necessary to exert astress on said mechanical connecting means; and wherein the mechanicalconnecting means comprise at least one strut whose shape is triangularwhen viewed in the direction of the force of gravity and where the baseof the triangle is embedded in the support and where the apex is joinedto the seismic mass.
 7. Accelerometer comprising a seismic mass capableof being subjected to a force induced by the acceleration to bemeasured, the seismic mass being connected to a support by mechanicalconnecting means capable of bending under the influence of said force,means of detection being provided to determine the acceleration inducedin the seismic mass by the force, means of compensation also beingprovided to compensate for the force exerted on the seismic mass bygravity, wherein the mechanical connecting means include at least onecomponent constituting said means of compensation while inducing in themechanical connecting means a prestress that counteracts the forceexerted on the seismic mass by gravity;wherein means for compensatingfor the force exerted on the seismic mass by gravity consist of amodification to the surface of the mechanical connecting means, thissurface modification inducing said prestress in the mechanicalconnecting means; and wherein said surface modification consists ofsurface doping of the material of the mechanical connecting means. 8.Accelerometer according to claim 7, wherein the mechanical connectingmeans being made of monocrystalline silicon, one of the surfaces of themechanical connecting means is doped using a dopant selected from thegroup that includes phosphorus, boron, xenon, titanium, arsenic andargon.
 9. Accelerometer according to claim 7, wherein surfacing dopingis carried out on opposite surfaces of the mechanical connecting means,each surface being processed with a different dopant, one of whichinduces a elongation stress while the other induces a compressive stressresulting by inducing a stress gradient in the thickness of themechanical connecting means that counteracts the force exerted on theseismic mass by gravity.
 10. Accelerometer according to claim 9, whereinthe mechanical connecting means being made of silicon, one of thesurfaces of the mechanical connecting means is doped using boron and theother with argon.
 11. Accelerometer comprising a seismic mass capable ofbeing subjected to a force induced by the acceleration to be measured,the seismic mass being connected to a support by mechanical connectingmeans capable of bending under the influence of said force, means ofdetection being provided to determine the acceleration induced in theseismic mass by the force, means of compensation also being provided tocompensate for the force exerted on the seismic mass by gravity, whereinthe mechanical connecting means include at least one componentconstituting said means of compensation while inducing in the mechanicalconnecting means a prestress that counteracts the force exerted on theseismic mass by gravity;wherein the mechanical connecting means compriseat least one strut; and wherein the seismic mass is supported by atleast a series of two struts arranged in opposing pairs in the seismicmass.
 12. Accelerometer according to claim 11, wherein the seismic massis supported by four upper struts arranged in opposing pairs, and byfour lower struts arranged in opposing pairs.
 13. Accelerometercomprising a seismic mass capable of being subjected to a force inducedby the acceleration to be measured, the seismic mass being connected toa support by mechanical connecting means capable of bending under theinfluence of said force, means of detection being provided to determinethe acceleration induced in the seismic mass by the force, means ofcompensation also being provided to compensate for the force exerted onthe seismic mass by gravity, wherein the mechanical connecting meansinclude at least one component constituting said means of compensationwhile inducing in the mechanical connecting means a prestress thatcounteracts the force exerted on the seismic mass by gravitywherein theseismic mass, the mechanical connecting means and the support constitutea one-piece assembly.
 14. Process for manufacturing an accelerometercomprising a seismic mass connected to a support by mechanicalconnecting means capable of bending under the effects of a force inducedin the seismic mass by an acceleration to be measured, means ofcompensation also being provided to compensate for the force exerted onthe seismic mass by gravity, including the following steps:masking ofone of the main surfaces of a substrate, or first surface, to demarcatethe seismic mass and the support, engraving the first surface of thesubstrate in the direction of the other main surface of the substrate,or second surface, leaving a membrane between the bottom of theengraving and the second surface, this engraving demarcating the seismicmass and the support, masking the second face of the substrate in such away as to mask the support, the seismic mass and the mechanicalconnecting means, engraving the second surface of the substrate to openunmasked areas of the membrane, processing at least part of the surfaceof the mechanical connecting means to induce a prestress counteractingthe force exerted on the seismic mass by gravity, thereby constitutingsaid means of compensation.
 15. Process according to claim 14, whereinsaid surface processing consists of a deposit of a material that createsa stress.
 16. Process according to claim 14, wherein said surfaceprocessing consists of doping.